replace ODIN_OS string with enums

vendor/stb/image/stb_image_write.odin

@@ -2,9 +2,9 @@ package stb_image
 
 import c "core:c/libc"
 
-when ODIN_OS == "windows" { foreign import stbiw "../lib/stb_image_write.lib" }
-when ODIN_OS == "linux"   { foreign import stbiw "../lib/stb_image_write.a"   }
-when ODIN_OS == "darwin"   { foreign import stbiw "../lib/darwin/stb_image_write.a"   }
+when ODIN_OS == .Windows { foreign import stbiw "../lib/stb_image_write.lib" }
+when ODIN_OS == .Linux   { foreign import stbiw "../lib/stb_image_write.a"   }
+when ODIN_OS == .Darwin  { foreign import stbiw "../lib/stb_image_write.a"   }
 
 
 write_func :: proc "c" (ctx: rawptr, data: rawptr, size: c.int)